Even if you are not too interested in bug fixes you should consider downloading and installing it because it also introduces new functionality. New features are listed separately in a PDF document. The feature list includes:

Export to CAD is now ArcView license tool

Support for WFS 1.0 (1.1 is already supported at 9.3)

Improved visibility of scale bar for web applications

Support for Windows Mobile Smartphone’s

Full support for editing data in PostGres SQL

Progress indicator for map and globe caching

Unfortunately, there was a one part of code that was not included in SP 1. That is Topology validation patch and it is available as separate download.

ESRI has released Service Pack 1 for all other 9.3 products. So now SP1 is available for: